An electric vehicle, a hybrid vehicle and a fuel-cell vehicle that obtain driving force by an electric motor incorporate a secondary battery. In an electric vehicle, electric energy stored in the secondary battery is used to drive the electric motor in order to drive the vehicle. In a hybrid vehicle, electric energy stored in the secondary battery is used to drive the electric motor in order to drive the vehicle, and the electric motor assists the engine to drive the vehicle. In a fuel-cell vehicle, electric energy from the fuel cell is used to drive the electric motor in order to drive the vehicle, and electric energy stored in the secondary battery is used in addition to the electric energy from the fuel cell to drive the electric motor in order to drive the vehicle.
Such a vehicle has a function of regenerative braking, which is a function of causing an electric motor to serve as an electric generator in braking of the vehicle, so that the kinetic energy of the vehicle is converted into electric energy to achieve braking. The electric energy converted here is stored in the secondary battery and reused in acceleration and the like.
As over discharge and overcharge of the secondary battery impair the performance of the battery and shorten the lifetime, it is necessary to know the state of charge (SOC, also referred to as remaining capacity) of the secondary battery to control charging and discharging. In particular, with respect to the hybrid vehicle where an electric generator is driven by a heat engine incorporated in a vehicle to generate electric energy which is then charged into a secondary battery, the amount of electric energy to be charged is often controlled to be an approximately intermediate state (50-60%) between a full-charge state (100%) and a no-charge state (0%), so that the secondary battery can accept regenerated electric energy and also can supply the electric motor with electric energy immediately on request. For such control and also to extend the lifetime of the secondary battery, it is necessary to avoid over discharge and overcharge.
A hybrid vehicle control apparatus incorporating an electric energy storage mechanism including such a secondary battery is disclosed in Japanese Patent Laying-Open No. 11-220810. The publication discloses a hybrid vehicle control apparatus that more properly provides control of an amount of electric energy to be regenerated when the vehicle decelerates, prevents deterioration of the electric energy storage device, and executes sufficient assistance in driving when it is necessary. The hybrid vehicle control apparatus controls a hybrid vehicle having an engine rotating a drive axle of the vehicle, a motor assisting the engine in rotating the drive axle with electric energy and having a regeneration function of converting kinetic energy of the drive axle into electric energy, and electric energy storage means for supplying electric energy to the motor and storing electric energy output from the motor. The control apparatus includes traveling state detecting means for detecting a traveling state of the vehicle including at least a vehicle traveling speed, remaining capacity detecting means for detecting a remaining capacity of the electric energy storage means, and decelerating regenerative control means for controlling an amount of electric energy to be regenerated by the electric motor when the hybrid vehicle decelerates based on the output of the traveling state detecting means. The decelerating regenerative control means includes means for correcting the amount of electric energy to be regenerated based on the output of the remaining capacity detecting means.
According to the hybrid vehicle control apparatus disclosed in the publication, the amount of electric energy to be regenerated by the motor when the hybrid vehicle decelerates is controlled based on the output of the traveling state detecting means, and the amount of electric energy to be regenerated by the motor is corrected based on the remaining capacity of the electric energy storage means. Therefore, the hybrid vehicle control apparatus can more properly provide control of an amount of electric energy to be regenerated when the vehicle decelerates, prevent deterioration of the electric energy storage device, and execute sufficient assistance in driving when it is necessary.
On the other hand, according to the hybrid vehicle control apparatus disclosed in the publication, the an amount of electric energy to be regenerated is corrected based on the remaining capacity of the electric energy storage means. Here, when the remaining capacity of the electric energy storage means is smaller than a first prescribed remaining capacity, or when it is smaller than a second prescribed remaining amount that is greater than the first prescribed remaining amount and a latest integrated discharged amount is greater than a prescribed discharged amount, a regenerated quantity increasing corrective coefficient (>1.0) is calculated in accordance with a vehicle speed, whereby a decelerating regenerative quantity is corrected to increase. In other words, when the remaining capacity of the electric energy storage means is small, the decelerating regenerative quantity is corrected to increase so that greater regenerative electric energy is obtained. If a secondary battery (in particular, a nickel-metal hydride battery) that is one of the electric energy storage means is continuously charged with great power for a long period during regenerative braking operation, the temperature of the battery increases, whereby the battery is deteriorated and its lifetime is shortened. Further, increase in the temperature of the battery may decrease an amount of electric energy to be fully charged. Thus, even when the energy is actually charged only for low SOC, it may be determined that the SOC is high. Then the regenerative power may not be accepted and the fuel economy may be impaired.
If the amount of electric energy to be charged is controlled to be limited after sensing a large amount of input electric energy, the vehicle is decelerated not through engine braking or wheel braking but through regenerative braking (a motor/generator generates electricity) until such limiting is started. Here, if the amount of electric energy to be charged into the secondary battery is limited as it is excessively great, from that time point the regenerative braking is limited and the vehicle is decelerated through engine braking or wheel braking. Here, the automatic transmission automatically downshifts the gear in order to more strongly exert engine braking in the middle of deceleration whereby the engine speed is increased, or a brake control computer more strongly exerts wheel braking. Thus, while the driver is depressing the brake pedal in the same manner, he/she may feel awkwardness.